The present invention relates to a capacitive pressure sensor. More specifically, the present invention relates to an improved filter for use with a capacitive pressure sensor.
FIG. 1A shows a sectional side view of a prior art ceramic capacitive pressure sensor 100. FIG. 1B shows an exploded view of sensor 100. Although sensors such as sensor 100 are well known, a brief description of its construction and operation will be provided. Sensor 100 includes a ceramic Pr body 102 (“Pr” representing “reference pressure”), a ceramic Px body 104 (“Px” representing “unknown pressure”), a thin, flexible ceramic diaphragm 106, and an inlet tube 108. As shown in FIG. 1A, when sensor 100 is assembled, Pr body 102 and Px body 104 are bonded together such that diaphragm 106 is clamped between the Pr and Px bodies. Diaphragm 106 may flex or deform in response to the pressure in inlet tube 108. Consequently, the pressure in tube 108 may be measured by detecting the position of diaphragm 106.
Pr body 102 and Px body 104 are shaped so that when they are bonded together, they define an interior volume. Diaphragm 106 divides this interior volume into an upper chamber 122 and a lower chamber 124 (the terms “upper” and “lower” and similar terms are used herein with reference to the drawings and do not imply any absolute orientation of the sensor). When sensor 100 is assembled, diaphragm 106 and Pr body 102 cooperatively define upper chamber 122, and diaphragm 106 and Px body 104 cooperatively define lower chamber 124. Px body 104 defines a central aperture 126. Inlet tube 108 also defines a central passageway 130, and passageway 130 is in fluid communication with the central aperture 126 of the Px body. Thus, passageway 130 is in fluid communication with the lower chamber 124.
Diaphragm 106 is a thin flexible ceramic disk onto which a conductive film 140 is deposited. Another conductive film 142 is deposited onto a central portion of Pr body 102 such that film 142 is spaced away from and opposite to the conductive film 140 on diaphragm 106. The two conductive films 140, 142 form two plates of a variable capacitor 144. As is well known, the capacitance provided by variable capacitor 144 varies with, among other things, the distance between the two plates 140, 142. Sensor 100 also includes conductive pins 150, 152. Pin 150 is electrically connected to the film 140 on diaphragm 106, and pin 152 is electrically connected to the film 142 on the Pr body 102. Pins 150 and 152 provide electrical connection to films 140 and 142, respectively, external to the body of sensor 100.
In operation, a reference pressure (e.g., vacuum) is established in the upper chamber 122 and the inlet tube is connected to a source of gas, the pressure of which is to be measured. Diaphragm 106 flexes, or deforms, in response to changes of pressure within the lower chamber, causing the capacitance provided by variable capacitor 144 to change in accordance with the pressure in inlet tube 108. Accordingly, the capacitance provided by variable capacitor 144 is indicative of the pressure within inlet tube 108.
As is well known, sensors such as sensor 100 often include additional features, which for convenience of illustration are not illustrated in FIGS. 1A and 1B. For example, such sensors often include a getter for maintaining a vacuum in the upper chamber 122. Also, such sensors often include two conductive films disposed on the Pr cover 102 instead of the single illustrated film 142. As is well known, having two such films allows the sensor to provide two variable capacitors instead of one, and this in turn can be used to improve the temperature stability of the sensor.
Pressure sensors such as sensor 100 are often used in integrated circuit fabrication foundries, for example, to measure the pressure of a fluid in a gas line that is being delivered to a deposition chamber, or to measure the pressure within the deposition chamber itself. Some of the processes used in integrated circuit fabrication, such as the etching of aluminum, tend to generate a large volume of particles or contaminants. It is generally desirable to prevent such contaminants from encountering the diaphragm 106. When such contaminants build up on diaphragm 106, the accuracy of the pressure measurement provided by sensor 100 is adversely affected. Accordingly, prior art pressure sensors have used a variety of mechanisms to prevent contaminants from reaching the diaphragm 106.
Although many such filtering mechanisms have been developed, there remains a need for improved methods and structures for preventing contaminants from reaching and settling on the diaphragm.